The major objective of this proposal is to investigate the external antigenic character of Toxoplasma gondii and its role in immunity. Because of the complex nature of this protozoan parasite little is known about its antigenic composition. Prior studies in this laboratory have demonstrated parasiticidal activity by certain hybridoma monoclonal antibodies against the tachyzoite of T. gondii. By using these and monoclonal antibodies specifically directed against the external parasite membrane yet to be generated I plan to identify various antigenic determinants of the tachyzoite (invasive form) as well as stage specific bradyzoite, sporozoite and gametocyte antigens. Monoclonal antibody resistant mutants will be isolated and used to define the antigenic character of the tachyzoite as well as the hosts immune response to particular antigenic sites. Antigen(s) selected for by monoclonal antibody will be purified by immunologic methods and chemically analyzed. Once the tachyzoite antigen(s) have been purified they can be used both as a vaccine in mice and as an experimental model for investigating the role of T-cell immunity in toxoplasmosis.